Hypercapnia (high pCO2) is observed in patients with lung diseases such as chronic obstructive pulmonary disease (COPD), cystic fibrosis, broncho-pulmonary dysplasia and advanced neuromuscular diseases. Also, a ventilation strategy that includes permissive hypercapnia is commonly used in mechanically ventilated patients with the acute respiratory distress syndrome (ARDS). Data from our group and others have challenged the existing paradigm suggesting that hypercapnia is beneficial for patients with ARDS. Instead, we have reported that elevated levels of CO2 have direct, deleterious effects on alveolar epithelial function and host immunity. Our published reports and data from preliminary studies in preparation for this proposal support our hypothesis that high levels of CO2 decrease alveolar epithelial cell migration and proliferation in the injured lung, thus, impairing lung injuy repair. Furthermore, we hypothesize that hypercapnia promotes the ubiquitin-proteasome mediated degradation of diaphragmatic myofibers and impairs the function of muscle satellite cells required for its regeneration. As such, in this application we propose to determine the mechanisms by which hypercapnia leads to impaired recovery from lung injury and the effect of high CO2 on diaphragm muscle degradation and impaired regeneration. We will test these hypotheses in three interrelated aims: in experiments pertaining to specific aim # 1, we will determine whether hypercapnia-mediated inhibition of GEFs-Rac1 leads to impaired cell migration; in experiments pertaining to specific aim #2, we will determine whether high CO2 via the miR-183/96/182 cluster impairs alveolar epithelial cell migration in vitro and lung injury repar in vivo; and in studies pertaining to specific aim # 3, we will determine whether hypercapnia leads to diaphragm dysfunction causing muscle degradation via the ubiquitin/proteasomal pathway and impaired muscle regeneration. We have conducted preliminary experiments for each of the specific aims which support the feasibility of this grant proposal. The proposed experiments will generate novel information on the mechanisms by which exposure to hypercapnia impairs alveolar epithelial and diaphragm function, which is of clinical significance for patients with ARDS who require mechanical ventilation and patients with chronic lung diseases and hypercapnia.